....I believe it was Mighty Mouse, or perhaps Super Mouse. At night all the characters from books and/or fairy tales emerged to play. A copy of FRANKENSTEIN falls over and the Monster rises to wreak havoc on the partying characters. Not to be confused with Frankenstein's Cat.
shareThere was a Gandy Goose cartoon called Mother Goose Nightmare (1945), but it doesn't feature Frankenstein's Monster. In this film Gandy and Sourpuss encounter the giant from "Jack and the Beanstalk."
https://www.youtube.com/watch?v=l5upku0feEs
Or you may possibly be thinking of the Warners Brothers Merrie Melodies cartoon Sniffles and the Bookworm (1939). This takes place in a bookstore at night and includes the Monster.
http://www.dailymotion.com/video/x24hnps_sniffles-and-the-bookworm-1939-leon-schlesinger_shortfilms